This invention relates to spark ignition engines in general and more particularly to spark ignition systems.
A conventional spark plug is adapted for insertion into an opening of an engine where an air-fuel mixture is present. This area is typically referred to as a cylinder or combustion chamber of the engine. Spark plugs are provided with an electrically insulating shell through which a high voltage electrode, also commonly referred to as the anode, extends into the combustion chamber. The high voltage electrode is connected to an ignition system which supplies a high voltage pulsating xe2x80x9cDC signalxe2x80x9d which is applied during each combustion cycle at a time when the piston is approaching the end of its upward motion and the valves are closed.
A second electrode is commonly referred to as the ground electrode or cathode. The ground electrode is typically a projection or protrusion extending inward from the shell of the spark plug and disposed in spaced apart relation with the high voltage electrode. The ground electrode is also disposed within the combustion chamber and is electrically common with the combustion chamber. The electrode separation distance is commonly referred to as an air gap or spark gap. The high voltage signal pulsating DC signal is sufficient to generate an electrical arc (or spark) across the air gap.
The spark generated quickly develops into a low impedance arc. The volume occupied by the arc is low, the reactivity of the arc is low and the electrode erosion rate is high. There is no external magnetic field or other device to cause the arc to move about or to otherwise increase in reactivity.
In systems well-known in the art, the spark gap is set prior to installation of the spark plug into a corresponding engine receptacle. Normally, the spark gap is adjusted to a distance to provide an arc having desired characteristics necessary for initiating proper combustion of the air-fuel mixture. Improper combustion can cause poor engine performance such as backfire and result in increased emissions of harmful pollutants such as NOx, unburned or partially oxidized hydrocarbons and CO.
Internal combustion engines which use spark plugs to ignite air-fuel mixtures are commonly referred to as spark ignition engines. Current spark ignition engines are commonly controlled to operate xe2x80x9cleanxe2x80x9d on fuel, operating at essentially the stoichiometric air/fuel ratio, in order to meet government imposed emission regulations. The stoichiometric ratio is the ratio of air/fuel required to completely combust the fuel. Most emissions generated by the combustion process are significantly reduced through use of a catalyst system positioned in the exhaust stream. The major role of the catalyst system is to reduce levels of NOx, unburned or partially oxidized hydrocarbons, and CO output by the combustion process. Thus, a careful control near the stoichiometric set-point is needed because the chemistry requires a reduction reaction to eliminate NOx while oxidation is required for elimination of unburned or partially oxidized hydrocarbons and CO.
An efficiency increase for internal combustion engines (estimated at up to 14-20%) could be realized if xe2x80x9clean-burnxe2x80x9d engines could supplant the current stoichiometric air/fuel engine technology. As used herein, lean-burn is the term used to describe an air/fuel mixture having excess air above the stochiometric air/fuel ratio. A major barrier to lean-burn engine use in the United States is the inability to meet the California and Federal emission standards. In particular, lean-burn engine mixtures have been shown to be unable to sufficiently suppress the generation of NOx during the combustion process. Once produced by the combustion process, current catalyst systems can only reduce NOx levels modestly ( less than 30%) from the levels generated from the combustion process.
Known strategies for reducing NOx formation in lean-burn engines include the use of exhaust gas recirculation. This method involves re-injecting combustion products back into the combustion chamber together with fresh air/fuel. A second strategy operates an engine very close to the lean-combustion misfire limit. The misfire limit occurs when combustion becomes erratic and generally incomplete.
Both of these strategies for reducing NOx formation during combustion are related. Both depend on dilution effects causing suppression of peak combustion temperatures. Thus, they could be used in combination. Pushing engine operation further into the lean regime permits greater potential efficiency gains. However, for lean-burn technology to become viable in view of strict emission standards, a method for suppressing emission of NOx and other environmentally harmful pollutants must be found.
Lean-burn mixtures can also result in ignition instability. The fuel injection and turbulent-mixing process inside the engine cylinders can create mixture stratification that can make ignition unreliable. This effect can become more pronounced for increasingly lean mixtures. Fluid volumes may be produced that are excessively lean to the point that flame propagation can become impeded. The fluid elements nearest the spark event can become particularly lean such that adequate flame kernel development is prevented even though the overall mixture stoichiometry is sufficient to otherwise sustain combustion.
Complete and partial misfires cause significant unburned fuel to be exhausted and engine performance to accordingly degrade. It estimated that up to 95% of the pollution emanating from a running combustion engine is generated during misfires. A misfire can also be followed by a relatively strong combustion event because the residual gases and recirculated gases contain unreacted fuel and oxygen. Thus, at a subsequent instant the air/fuel mixture may have more fuel and air than the engine set-point would otherwise allow. This stronger combustion event can result in a higher combustion temperature than is meant to occur and is likely to produce relatively high quantities of NOx. This general cycle-to-cycle variation in combustion events has been a major focus of engine research. Tolerable levels of misfire are generally accepted to be limited to 1-5 misfires per 1000 combustion events.
Some principles of high-pressure (10 bar) spark discharges are presented to aid in an understanding of the invention. High-pressure sparks have properties which differ from low-pressure (but still collision dominated) sparks. In low pressure sparks, a Townsend discharge may occur where ambient free electrons are accelerated by an electric field and ionize neighboring gas particles through collisions. This is known as electron impact ionization. Newly generated xe2x80x9csecondaryxe2x80x9d electrons are themselves accelerated by the ambient electric field causing an avalanche of electron and positive ion production. In low pressure discharges, the avalanche grows at the electron drift velocity, while plasma densities and associated currents are relatively low and collisional diffusion is usually significant.
At high pressures, such as 10 bar, the plasma charge density may build up to much higher values compared to the charge density normally built up at low pressure (e.g. 1 bar). As a result, the mutual coulomb or space charge forces, are much stronger at high pressures than the vacuum electrostatic forces. Ionization in this case produces an almost perfectly space charge neutralized plasma. However, the coulomb forces due to the residual space charge still dominate the forces due to the applied fields. The resulting space charge shielding of the applied fields by the plasma causes the electrical fields within the forming spark to be quite low.
According to Gauss""s Law, this charge configuration makes the electrical field between the emerging spark and the spark plug anode correspondingly higher. This process continues during the ionization avalanche with the electric field in the front of the plasma, commonly referred to as the plasma front, becoming progressively stronger with time. For example, FIG. 1 shows an electrical potential distribution after approximately 1 or 2 nsec after an avalanche has been initiated. This spark phase may be characterized as the breakdown phase. During this phase, regions of high electrical field intensity 100 located between the anode 102 and plasma front 104 correspond to a region having a large gradient in the electrical equipotential lines 106. Regions of high electrical field intensity 100 correspond to regions where auto-ionization is probable.
During the first nanosecond or so of each combustion cycle, the plasma front 104 moves quickly towards anode 102, as a result of high levels of electron impact and photon ionization. The electron temperature may also be increased during this process. This avalanche process differs from the low pressure case in that the speed of propagation of the plasma front 104 can be orders of magnitude greater than the electron drift velocity since photon induced ionization effects can become dominant.
FIG. 2 shows an arc 101 and the resulting equipotential distribution 106 at a time in the combustion cycle later than that shown in FIG. 1. For example, 10 ns or more after the breakdown avalanche. At this point, the breakdown phase has ceased. Velocities of charged particles 112 are indicated by the relative length of the tail associated with each charged particle (squares). The arc 101 does not reach the cathode 108 due to the cathode sheath. The resulting discharge has a high conductivity and develops into a low voltage, high current arc. If this arc were stable it would likely produce less chemical reactivity within it since the electron temperature would be much lower due to the low electric fields because of significant levels of plasma shielding evident from FIG. 2.
During this post breakdown phase, arc and glow discharges can result. Both arc and glow phases produce limited reactivity, with most reactivity occurring near the cathode sheath 110 which is located between the plasma front 104 and the cathode 108. In and near the cathode sheath, the electrical field is relatively higher than other regions of arc 101 and is correspondingly more highly reactive. However, even the reactivity around cathode sheath 110 is substantially less than the region of high electrical field intensity 100 shown in FIG. 1 provided during the short interval in each combustion cycle that comprises breakdown phase (approximately 1 nsec).
Two significant concerns relate to the ability of a combustion engine modeled as a high pressure spark to ignite the fuel. First, the volume occupied by the narrow spark channel is quite low, perhaps 0.003 mm3. Second, the electron temperatures in the arc phase are the lowest, and as a result reaction probability is relatively low. Thus, in order to increase the probability for a fuel ignition event to occur one can attempt to increase the volume occupied by the spark and/or attempt to increase the electron temperature in the arc phase.
Increasing the probability of ignition could provide low emission operation under conditions such as increasingly leaner fuel regimes. This combination could improve fuel economy without a corresponding degradation in engine performance and increase in harmful emission products such as NOx.
A spark plug improvement is noted in SAE 760764 by D. J. Fitzgerald of the Jet Propulsion Laboratory. A generated arc is caused to move by Jxc3x97B induced magnetic fields. The magnetic fields are induced from the arc current itself. In this manner, the arc is made to cover a larger volume than a standard spark plug embodiment. However the arc current used is many orders of magnitude larger (10,000 Amps) than standard spark plugs in order to provide a sufficient Jxc3x97B force to move the arc. A power supply large enough to produce the required arc current would not be practical in motor vehicles. Moreover, high arc currents increase electrode erosion rates which reduce spark plug lifetimes. Moreover, high arc currents are known to adversely impact combustion efficiency.
Tozzi, U.S. Pat. Nos. 5,555,862 and 5,619,959 (Tozzi inventions or ""862 and ""959, respectively), each disclose use of one or more permanent magnets to provide adjustable length spark gaps. In the Tozzi inventions, arcs produced can be moved by application of variable levels and durations of electrode current applied to the high voltage electrode. Based on Tozzi""s disclosed electrode configuration and relative positioning, different arc positions result in different spark gap lengths. Magnets are used to reduce the amount of electrode current required to position an arc in a desired position between the electrodes. Thus, Tozzi""s magnets are arranged so that a radial magnetic field is established in the area of the air gap to help propel the arc outwardly (axially) from the spark plug cavity to achieve a user desired spark gap length (see FIG. 4 in ""862).
In addition, arcs produced by Tozzi generally have a fixed azimuthal orientation having no rotation component. Thus, Tozzi""s arc does not expose relatively large volumes of ignitable fuel mixtures to the arc. This reduces the probability of ignition compared to an arc having a varying azimuthal orientation. Tozzi is also subject to anode erosion at breakdown, since breakdown occurs over a small area. Moreover, Tozzi""s insulator and electrode configurations result in breakdown occurring largely parallel to magnetic field lines which can cause catastrophic breakdowns which can result in damage to the insulators, which can render an ignition system inoperable.
Cylindrical Coordinate System
Cylindrical coordinates are a generalization of two dimensional polar coordinates to three dimensions by superposing a height (denoted z) axis on the polar axis. In this application, (r, xcex8, z) is normally used. The radial distance is denoted as r, the azimuthal angle xcex8 and the height, axial component or cylindrical axis, z.
Lorentz Force
A Lorentz force is exerted on charged particles moving in regions where a magnetic field is oriented perpendicular to the particle""s velocity. In such a situation, the magnetic force serves to move the particle in a circular path. According to the xe2x80x9cright hand rulexe2x80x9d applicable for positively charged (and xe2x80x9cleft hand rulexe2x80x9d for negatively charged) particles, the magnetic force acting on the charged particle always remains perpendicular to the charged particle""s velocity. The magnitude of the magnetic force is:
F=q Vxc3x97B
where q is the magnitude of the charge of the charged particle, V its velocity (for collision dominated transport the velocity may be replaced by the mean or drift velocity and the force then becomes the mean force), and B is the magnetic field and xe2x80x9cxxe2x80x9d is the vector cross product of B and V. Magnetic flux density relation to magnetic scalar potential:
The basic laws of magnetostatics are:
∇xc3x97B=4xcfx80J/c
∇xc2x7B=0
Where J is the current density, B is the vector magnetic induction (or the magnetic flux density) and c is the speed of light. If the current density is zero in the region of interest, ∇xc3x97B=0 permits the expression for B to be written simply as the gradient of a magnetic scalar potential; B=xe2x88x92∇xcfx86m.
An arc utilizing device includes a first electrode, a second electrode electrically insulated and disposed radially outward from the first electrode. The electrodes form a gap region across which an arc can be established. The arc utilizing device also includes a structure for modification of the arc, the modification including rotation of the arc.
A spark plug device includes a substantially electrically insulating shell, a first electrode situated substantially within the shell, the first electrode having a length protruding from the shell defining an axis for rotation. A second electrode is disposed radially outward from the first electrode, the electrodes forming a gap region across which an arc can be established. The spark plug includes a structure for modification of the arc, the modification including rotation of the arc.
The structure for modification can be adapted for oscillating an output of the arc and can include at least one magnet which may be a permanent magnet. The first electrode can include a broadened tip for at least a portion of the first electrode length within the gap region, the broadened length having larger cross sectional areas relative to cross sectional areas adjacent to the gap region.
The arc can rotate in a path substantially around the axis for rotation. The structure for modification can provide a magnetic field oriented substantially parallel to the axis for rotation, whereby an electric field in the gap region generated from an electrical potential applied between the electrodes is oriented substantially radially, or perpendicular to the magnetic field. The structure for modification can provide a magnetic field in the gap region of from approximately 0.05 to 1 Tesla. The gap region can be substantially annular. The electrode spacing can be approximately 0.5 mm to 4 mm in the gap region. The applied electrical potential can be from approximately 5 kV to 80 kV. The magnet can be at least one electromagnet which can be used to also provide a pulsed electrical field between the electrodes.
A method for operating a spark plug device includes the steps of providing a spark plug device having a substantially electrically insulating shell, a first electrode situated substantially within the shell, the first electrode having a length protruding from the shell defining an axis for rotation. A second electrode is disposed radially outward from the first electrode, the electrodes forming a gap region across which an arc can be established. The method includes the step of modifying the arc, the modifying including rotating the arc. The method can further comprise the step of oscillating an output of the arc.
The spark plug can include at least one magnet for modifying the arc which may be a permanent magnet. Rotation can be at least in part around the axis for rotation, produced by at least one magnet generating a magnetic field oriented substantially parallel to the axis for rotation. Accordingly, an electric field in the gap region generated from an electrical potential applied between the electrodes can be oriented substantially radially, or perpendicular to the magnetic field.
At least one magnet can generate a magnetic field strength in the gap region of approximately 0.05 to 1 Tesla. The gap region can be substantially annular. The electrode spacing can be approximately 0.5 mm to 4 mm in the gap region and the applied electrical potential difference can be from approximately 5 kV to 80 kV.
A method for operating a combustion engine includes the steps of providing a spark plug device having a substantially electrically insulating shell, a first electrode situated substantially within the shell, the first electrode having a length protruding from the shell defining an axis for rotation. A second electrode is disposed radially outward from the first electrode, the electrodes forming a gap region across which an arc can be established. The method includes modifying the arc, wherein the arc modifying includes rotating the arc and operating the combustion engine to produce combustion.
The method can further comprise the step of oscillating an output of the arc. The method can include the step of providing the spark plug with at least one magnet for modifying the arc. The at least one magnet can be a permanent magnet. The rotation can be at least in part around the axis for rotation, the at least one magnet generating a magnetic field oriented substantially parallel to the axis for rotation. Accordingly, an electric field in the gap region generated from an electrical potential applied between the electrodes can be oriented substantially perpendicular to the magnetic field.
At least one magnet can generate a magnetic field strength in the gap region of from approximately 0.05 to 1 Tesla. The gap region can be substantially annular having a nearly constant electrode spacing throughout. The electrode spacing can be approximately 0.5 mm to 4 mm in the gap region and the applied electrical potential can be from approximately 5 kV to 80 kV. Operating the combustion engine produces combustion levels of NOx which are reduced compared to NOx levels generated by combustion engines using conventional spark plugs. Operating the combustion engine also can produce levels of NOx which are reduced compared to NOx levels generated by combustion engines using conventional spark plugs. In addition, the fuel efficiency of the combustion engine can be enhanced compared to combustion engines which use conventional spark plugs. The method of operating a combustion engine can further include the step of supplying a lean-burn fuel mixture to the combustion engine which can be an air to fuel ratio of from approximately 20:1 to approximately 100:1.
A combustion engine includes at least one cylinder for receiving a combustible fuel mixture therein. A spark plug combusts the combustible fuel mixture, the spark plug including a first electrode situated substantially within a shell. The first electrode has a length protruding from the shell defining an axis for rotation. A second electrode is disposed radially outward from the electrode, the electrodes forming a gap region across which an arc can be established. The combustion engine also includes a structure for modification of the arc, the modification including rotation. The output of the arc can oscillate. The structure for modification can include at least one magnet. The at least one magnet can be a permanent magnet. The rotation can be at least in part around the axis for rotation, with at least one magnet generating a magnetic field oriented substantially parallel to the axis for rotation, whereby an electric field in the gap region generated from an electrical potential applied between the electrodes is oriented substantially perpendicular to the magnetic field.
At least one magnet can provide a magnetic field strength in the gap region of from approximately 0.05 to 1 Tesla. The gap region can be substantially annular. The electrode spacing can be approximately 0.5 mm to 4 mm in the gap region and the applied electrical potential can be from approximately 5 kV to 80 kV. The combustible fuel mixture can be a lean-burn mixture which can be an air to fuel ratio of from approximately 20:1 to approximately 100:1.